runtime: use 4K as the boundary of legal pointers

Currently, the check for legal pointers in stack copying uses
_PageSize (8K) as the minimum legal pointer. By default, Linux won't
let you map under 64K, but

1) it's less clear what other OSes allow or will allow in the future;

2) while mapping the first page is a terrible idea, mapping anywhere
above that is arguably more justifiable;

3) the compiler only assumes the first physical page (4K) is never
mapped.

Make the runtime consistent with the compiler and more robust by
changing the bad pointer check to use 4K as the minimum legal pointer.

This came out of discussions on CLs 34663 and 34719.

Change-Id: Idf721a788bd9699fb348f47bdd083cf8fa8bd3e5
Reviewed-on: https://go-review.googlesource.com/34890
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/ssa/nilcheck.go

@@ -132,6 +132,8 @@ func nilcheckelim(f *Func) {
 }
 
 // All platforms are guaranteed to fault if we load/store to anything smaller than this address.
+//
+// This should agree with minLegalPointer in the runtime.
 const minZeroPage = 4096
 
 // nilcheckelim2 eliminates unnecessary nil checks.

src/runtime/malloc.go

@@ -157,6 +157,13 @@ const (
 	_MaxGcproc = 32
 
 	_MaxArena32 = 1<<32 - 1
+
+	// minLegalPointer is the smallest possible legal pointer.
+	// This is the smallest possible architectural page size,
+	// since we assume that the first page is never mapped.
+	//
+	// This should agree with minZeroPage in the compiler.
+	minLegalPointer uintptr = 4096
 )
 
 // physPageSize is the size in bytes of the OS's physical pages.

src/runtime/stack.go

@@ -601,7 +601,7 @@ func adjustpointers(scanp unsafe.Pointer, cbv *bitvector, adjinfo *adjustinfo, f
 			pp := (*uintptr)(add(scanp, i*sys.PtrSize))
 		retry:
 			p := *pp
-			if f != nil && 0 < p && p < _PageSize && debug.invalidptr != 0 {
+			if f != nil && 0 < p && p < minLegalPointer && debug.invalidptr != 0 {
 				// Looks like a junk value in a pointer slot.
 				// Live analysis wrong?
 				getg().m.traceback = 2